There are systems that involve a tracking function performed by a centralized and stationary unit. Examples of such systems are shown in U.S. Pat. Nos. 5,594,425; 5,312,618; and 5,043,736. In the usual case, a target's location information is transmitted to a monitoring station where the information is processed and actions taken accordingly. A variation to this is shown in U.S. Pat. No. 5,389,934 which allows the searcher/controller to be mobile; where in one embodiment the system described is a rover which when called by a telephone would become active, relay a voice description of its location and then become inactive; and another embodiment provides an arrow visual to point in the direction of the target, but does not take advantage of the specific spatial relationship calculations used in the present invention.
Currently there is under consideration and implementation a concept referred to as E911 which contemplates that a cell phone user can make a “911” emergency call and a system will exist with a location technology that will enable locating the cell phone without any need to have the user interact. The location technology, the cell phone network and the cell phone (sometimes referred to as a handset) will enable the cell phone's location to be determined and communicated to a responding emergency service. Numerous location technologies and standards are presently under consideration and implementation in conjunction with wireless networks in general and cellular networks in particular not only for implementation of the E911 capability, but for other purposes as well.
Many papers, announcements and reports describe these location technologies, how they function with wireless networks, in particular cellular networks, their pros and cons, and the ways in which they can provide useful services. A simple Internet search will suffice to find many such publications. Some of them are:
“Geolocation and Assisted-GPS”, Goran M. Djuknic and Robert E. Richton, a white paper published by Lucent Technologies-this document has a particularly helpful description of the various geolocation technologies;    “Location Technologies for GSM, GPRS and UTMS Networks”, a White Paper by SnapTrack, A QUALCOMM Company;    “Geolocation and Assisted-GPS”, by Djuknic and Richton, Bell Laboratories, Lucent Technology published in Computer magazine, February 2001;    “Assisted GPS: A Low-Infrastructure Approach”, by LaMance, DeSalas and Jarvinen, in GPSWorld, Mar. 1, 2002;    “An Introduction to SnapTrack Server-Aided GPS Technology”, by Mark Moeglin and Harvey Krasner, published on Aug. 10, 2001 as a White Paper;    “Satellite-Based Positioning Techniques”, by Jari Syrjarinne, published by Nokia.    “Hybrid Wireless Assisted GPS Architecture” a publication by SnapTrack;    “A Position Determination Service Standard for Analog Systems” published by TIA/EIA, Jun. 6, 2000 (see also TIA/EIA-553-A);    “Position Determination Service Standard for Dual Mode Spread Spectrum Systems” publication of TIA as standard 3GPP2 C.S0022-0-1;    “High performance wireless location technology” a news release by TruePosition, Inc.;    “LOCATION TECHNOLOGIES FOR ITS EMERGENCY NOTIFICATION AND E911”, By Robert L. French and Clement J. Driscoll a paper prepared for ION National Technical Meeting, Santa Monica Calif., Jan. 22-24, 1996. The content of all of these references is incorporated by reference into this specification.
Also an Internet search for the relevant standards bodies such as TIA's 3GPP (principally dealing with GSM networks) and 3GPP2 (dealing with CDMA networks) will provide information about the process of evolving technical standards. With such helpful descriptive material available it will not be necessary to provide exhaustive descriptions except to the extent necessary to understand the configurations of systems and operation of methods of the present invention.
Some of the location technologies are; Assisted GPS (AGPS), network wireless triangulation using Time Difference of Arrival (TDOA), Advanced Forward Link Trilateration (AFLT), Enhanced Observed Time Difference (EOTD), and/or Angle of Arrival (AOA) as well as hybrid solutions such as AGPS augmented with other location technologies that make use of all available measurements be they satellite or land based signals. For the purposes of this specification and claims the phrase: “land based location technology” or “land-based location technology source” will be used to define any location technology that makes use of measurements obtained from land based transmitters to form a position solution; such as: AFLT, TDOA, EOTD, AOA, or UTDOA. Also, for purposes of this specification and claims, the phrases “satellite based location technology” or “satellite-based location technology source” will be used to define any location technology that makes use of data transmitted from satellites in satellite-based positioning systems to form a position solution. Such satellite-based positioning systems include, but are not limited to: the Global Positioning System (GPS), the Global Navigation Satellite System (GLONASS), and the GALILEO satellite radio navigation system (GALILEO). Further, the phrase “location technology source” as used herein refers to a system utilizing a particular type of location technology. Finally, except if the context clearly indicates otherwise, the words “position” and “location” are used interchangeably and are considered synonymous.
As indicated above, location or position systems may be satellite-based or land-based systems. Typically, these systems comprise one or more location measurement data sources, (for example, radio positioning entities such as satellites, in the case of satellite-based systems, or land-based transmitters, in the case of land-based systems) that provide data which a location subject, that is a unit whose location is being sought (for example, a cell phone with a GPS receiver) uses to make position and other navigation data measurements. Further, although a location subject will receive measurement data needed to determine its position, the calculation of position and other use of the data may be done elsewhere, in which case the location subject may transmit or otherwise relay the measurement data to another location subject or other device or system which will perform the calculations necessary to determine the location of the location subject. Other location measurement data sources may not be radio positioning entities, but may still provide data used to make position and navigation data measurements. Another example of a location measurement data source may be an optical entity that provides position information to a location subject. As used herein, a location technology source comprises one or more location measurement data sources.
Units having a GPS receiving capability may use low cost GPS receiver chipsets that come in two basic architectures. These two types of architectures are generally referred to as a closed-loop receiver architecture and an open-loop receiver architecture. The two types of architectures can be distinguished by the complexity of the architectures and the positioning accuracy provided.
A closed loop receiver is a receiver that creates a complete replica of the received signal through a reconstruction process that is synchronized to the incoming signal through a tracking process. All measurements, both code pseudorange and carrier phase, are made on the reconstructed signal. The tracking process is based upon a coherent comparison (containing both amplitude and phase information) of the reconstructed signal and the receive signal. The basic limitation of a closed loop receiver is the signal level at which the signal can no longer be tracked. For carrier phase tracking, the theoretical threshold for GPS is in the mid twenties in CNo dbHz terms. For carrier frequency tracking, the performance is usually limited by the quality of the GPS reference oscillator, but for most crystal based oscillators this limit falls in the high teens in CNo dbHz terms.
A closed loop receiver produces extremely good measurements, both position (pseudorange) and velocity (carrier). In addition, the pseudorange can be averaged for very long time since all platform dynamics and satellite dynamics can be eliminated by technique referred to as Carrier Aided Smoothing (CAS), since the only non-constant term left in the code tracking result is the ionosphere divergence between code and carrier. Since this divergence occurs very slowly, long averaging times can be applied. Because the pseudorange measurements can be integrated for a very long time, all but the very close in (slowly changing) multipath errors can be reduced significantly.
Closed loop receivers typically achieve relative positioning results, based upon pseudorange measurements, approaching sub-meter performance. The velocity performance, which is based upon the carrier phase measurements, can obtain cm/sec. The obvious limit to closed loop receivers is the tracking thresholds of either the carrier phase-tracking or carrier frequency-tracking loop. The CNo presented above, as tracking thresholds, are typical of a signal obscured by dense foliage or light indoor signal reception, assuming reasonable antenna performance. However, reasonable antenna performance is not necessarily a valid assumption in a cellular handset, where antenna/receiver implementation loss can approach 6-7 dB.
An open loop receiver creates an approximate replica of the received signal, but does not attempt to synchronize this replica through a tracking process. Measurements are based upon power maximization algorithms. The primary process employed by these receivers is one of searching for the satellites in a number of possible time-frequency locations. The pseudorange measurement is derived from the time bin with the maximum power; although some interpolation between adjacent bins containing high-power measurements may be used to refine the estimate of pseudorange. The Doppler measurement is derived from the frequency bin with the largest detected power. The maximum power candidates are produced by coherently integrating the complex signal for maximum of 20 milliseconds, which is the Binary Phase-Shift Keying (BPSK) data rate of the signal and limits the length of coherent integration. Further integration can be performed after the complex signal is turned into a power measurement in order to achieve greater sensitivity; or the ability to discern a signal presence in a time-frequency bin at very low CNo. Many manufacturers of these receiver types claim sensitivity down to the low teens in CNo dbHz terms.
Typically, receivers of this type are designed to achieve an acceptable level of performance and not an optimum level performance. This target has resulted in performance levels inferior to closed loop receivers, even in clear sky, high signal level environments. The goal is to provide a set of measurements that will produce a position solution that achieves the stated requirements of the FCC in the E911 mandate. They are not designed to produce continuous measurements that can be utilized to calculate a robust navigation solution, including not only position but also velocity. Furthermore, since open loop receivers typically produce unrelated measurements, the point solutions derived can be contaminated by anomalous behavior resulting from phenomena like multipath. Without some consistent measurement history, it becomes very difficult to detect such anomalous behavior.
Open loop receivers achieve absolute positioning performance of greater than 10 meters in clear sky environments, and much worse, hundreds of meters, in signal obscured environments like urban canyons. The relative performance of open loop receivers is not much better than the absolute performance, since no averaging of measurements or position is employed. The main advantages of open loop receivers are that the time required to gather a measurement suite is usually shorter than a closed loop receiver and that measurements can be generated at signal levels that cannot be tracked by closed-loop receivers.
Both closed loop and open loop receivers may use the location assistance techniques described above. GPS receivers typically found in cellular handsets are of the open-loop variety, which meet the E911 positioning mandates, but do not provide a high level of position accuracy, especially for receivers in motion.